The invention relates to a method and device for determining the quantity of dispersed solid material in a liquid.
In various chemical and biochemical processes, solid materials are formed which are dispersed in a reaction liquid. For example, in biotechnological fermentation processes, microorganisms are cultivated as a dispersed culture in a fermentation broth. To effectively regulate such processes, a variety of physical measurements may be used, including oxygen content, degree of acidity, carbon dioxide content, temperature and content of other substances. In particular, it is often of importance to determine the quantity of solid substances. For example, in the case of a fermentation process, microorganisms, and/or substances excreted by microorganisms, which are contained in the reactor vessel, are to be determined.
One approach to measuring the level of solid materials involves the periodic removal of samples from the vessel and the determination of the quantity of solid substance in the samples by chemical, electrical, microscopic or photometric methods, or by weighing. Taking samples, however, is beset with a number of drawbacks. A relatively large amount of time is lost between taking a sample and the measurement of solid material therein, such that the information obtained may be unreliable or is no longer relevant to the control of the process. Moreover, taking a sample is risky in certain cases because undesirable microorganisms may be introduced into the reactor vessel. There is therefore a strong preference for carrying out measurements in the reactor vessel itself, rather than on removed samples.
In order to obtain satisfactory results for measurements in the vessel, the method of measurement and the measuring device will have to satisfy a number of conditions. The sensor must be biologically and/or chemically inert and must be capable of being sterilized at approximately 120.degree. C. It must be possible to avoid or correct contamination and build-up of solid constituents on interface surfaces which play a part in the measurement.
Further, it must be possible to achieve a reasonable measurement result even with a relatively high density of the dispersed solid constituents, and the sensor must be able to function in coloured and turbulent solutions. Finally, it is desirable to keep the cost of the sensor down.
It is an object of the invention to provide a method and a device for determining the quantity of dispersed solid material in a liquid which satisfies these conditions.